The present invention relates to valve assemblies, e.g., for use in connection with fluid-filled storage tanks, such as hydrocarbon product storage tanks. More particularly, the invention relates to valve assemblies which are useful in types of service in which the valve pallet or pallets are repeatedly seated and unseated, such as breather valves.
Maintaining the pressure in a fluid-filled storage tank within the desired operating range reduces the risk of damaging the tank and/or fire and/or explosion. One approach to achieving this beneficial goal is the use of a breather valve assembly mounted to the tank. Such breather valves often include a movable pressure pallet which allows excess pressure in the tank to be relieved and a movable vacuum pallet which allows atmospheric air to enter the tank to relieve any undesirably reduced pressure in the tank.
Such breather valve assemblies differ substantially in structure and use from emergency vents which are also often mounted on fluid-filled tanks. For example, whereas the breather valves include means for relieving both excess pressure and reduced pressure in the tank, the emergency vents provide only excess pressure relief. Also, the orientation of the movement of the pressure pallet away from and toward the valve seat in a breather valve assembly is controlled or guided quite rigorously so that the pressure pallet and valve seat are effectively and quite precisely aligned, and therefore, can be effectively sealed in spite of frequent and repeated movements of the pressure pallet away from and toward the valve seat. Also, the pressure pallet may be lifted only a small distance away from the valve seat. Thus, even with the tank "breathing" out excess pressure, the pressure pallet may be in close proximity in the valve seat. In contrast, an emergency vent is designed for relatively infrequent operation. When operated, the vent cover should be removed from the vent outlet as soon and as completely as possible. In addition, after operation of an emergency vent, it is often inspected to be sure the vent has returned to its closed position and is ready for further use. A breather valve may operate, i.e., the pressure pallet moves away from and toward the valve seat, many times before being inspected. In effect, a breather valve involves frequent relatively small, controlled movements of the pressure pallet relative to the valve seat whereas an emergency vent involves infrequent operation in which the vent cover is moved quickly and completely away from the vent outlet. Thus, while a hinge mechanism is often used in a emergency vent to allow quick and complete movement of the vent cover away from the vent outlet, such a mechanism is not preferred for controlling the orientation of the relative subtle movements of a pressure pallet vis-a-vis the valve seat in a breather valve assembly.
The differences between a breather valve assembly and an emergency vent are so substantial that it is not appropriate to use one's knowledge of one system to modify the other system.
A breather valve assembly is often set, e.g., using weights, springs or the like, to open at a predetermined pressure condition in the tank. In order to conserve the product in the tank and to avoid excessive emissions from the tank, the breather valve assembly should open at the predetermined pressure condition in the tank and should seal tightly after the pressure condition in the tank has been relieved. Many of the breather valve assemblies currently in use, e.g., which utilize conventional seals, such as washer-type seals which contact both the valve seat and the pressure pallet at the area of seal/seat contact, disadvantageously lose the ability to open at a predetermined pressure condition in the tank and seal tightly. After a period of use, these breather valves tend to allow the passage of fluid from the tank at all times or at least at tank pressures significantly below the predetermined tank pressure for the valve to open. Also, because the valves are repeatedly opened and closed, the seals tend to lose their effectiveness, which also leads to product leakage. A breather valve assembly with a new sealing mechanism would be advantageous.